One bus system is the Controller Area Network (CAN). It is described, for example, in the BOSCH CAN specification 2.0, which may be downloaded from the Robert Bosch GmbH website http://www.semiconductors.bosch.de. The CAN protocol is widely common in the automobile industry, in industrial automatization or also in building networking systems.
The bus system in such cases is usually a cable pair such as, for example, a twisted copper cable. Communication users connected to the bus system, also called CAN nodes, may exchange messages via the bus system, which contain data to be communicated, for example. The messages are constructed according to the rules predefined by the CAN protocol.
The messages to be transmitted in the CAN protocol include a header, a data field and a final section, the data to be transmitted being included in the data field. The header of the message includes a Start-of-Frame bit an Arbitration field and a Control field. The Arbitration field includes the identifier, which determines the priority of the message. CAN supports identifier lengths of 11 bits (“Standard Format” or “Base Format”) and 29 bits (“Extended Format”). The Control field includes a Data Length Code, which predefines the length of the data field. The final section of the message includes a CRC field, an Acknowledge field and an End-of-Frame field. This CAN protocol is hereafter referred to as “Classic CAN”. CAN bit rates up to 1 Mbit/s are achieved via Classic CAN.
The individual bits are usually represented as voltage differences between the lines of the bus system, a first voltage difference corresponding to a logic “0” and a second voltage difference corresponding to a logic “1”. In CAN the first voltage difference, which corresponds to a “0”, is also referred to as “dominant level”, and the second voltage difference, which corresponds to a logic “1”, is referred to as “recessive level”.
The roles of sender and receiver for the messages to be transmitted are assigned among the users by an arbitration method based on information from the header of the messages. Arbitration method in this context means that, based on an identifier included in the message, it is negotiated which user obtains transmit access to the bus if multiple users attempt to send a message simultaneously, precisely one user being awarded transmit access by the arbitration method in the case of uniquely assigned identifiers.
The introduction of increasingly networked applications, for example, of assistance systems in vehicles, or of networked control systems in industrial plants, results in the general requirement that the bandwidth for serial communication must be increased.
Another protocol, called the “CAN with Flexible Data Rate” or CAN FD, is also known. This protocol uses the bus arbitration method known from the CAN, but increases the bit rate by switching to a shorter bit duration after the end of arbitration up to the Bit CRC Delimiter. The effective data rate is also increased by allowing longer data fields.
CAN FD may be utilized for general communication, but also in specific operating modes, for example, software download or End-of-Line programing or for maintenance work.
Two sets of bit clock configuration registers are usually provided in CAN FD, which define a bit duration for the arbitration phase and another bit duration for the data phase. The bit duration for the arbitration phase has the same restrictions as in Classic CAN networks, the bit duration selected for the data phase may be shorter with respect to the efficiency of the selected transceivers and the requirements of the CAN FD network.
A CAN FD message is made up of the same elements as a Classic CAN message, but which differ in detail. Thus, in a CAN FD message, the data field and the CRC field may be longer. Examples of Classic CAN and CAN FD messages are depicted in FIG. 1.
CAN FD supports both identifier lengths of the CAN protocol, the 11 bit length “Standard Format”, which is also called “Base Format”, and the 29 bit long “Extended Format”. CAN FD messages have the same structure as Classic CAN messages. Classic CAN messages and CAN FD messages are distinguished by a control signal, namely, a reserved control bit, which is always transmitted as dominant in the Classic CAN, bears the name “r0” or “r1”, and is located in the control field in front of the Data Length Code. In a CAN FD message, this control bit is transmitted as recessive and is called FDF (“FD Format”).
Many other communication systems are acquainted with similar control signals or control bits, by which, for example, different formats may be differentiated. The inventive idea is described below based on CAN. However, the present invention is not limited as a result to CAN bus systems, but may be implemented based on all bus systems, which satisfy the features of the definition of the species of the method described herein.
In the CAN FD messages, unlike the Classic CAN messages, additional control field bits follow, for example, the BRS bit, which indicates the position at which, provided the BRS bit has a corresponding value, the bit duration in a CAN FD message is switched to a shorter value. This is depicted in FIG. 1a by arrows, which divide the messages in a section referred to as “CAN FD Data Phase”, in which the high bit rate or the short bit duration is used, and in two sections named “CAN FD Arbitration Phase”, where the lower bit rate or the longer bit duration is used.
The number of bytes in the data field is indicated by the data length code. This code is 4 bits wide and is transmitted in the control field. The coding is different in CAN FD than in the Classic CAN. The first nine codes (0x0000 through 0x1000) are the same, but the following codes (0x1001 through 0x1111) correspond to larger data fields of the CAN FD messages, for example, 12, 16, 20, 24, 32, 48 and 64 bits.
Transceivers are bus connection units, which when transmitting, convert the logic signals, which are received, for example, by a communication controller or microcontroller, into the physically provided signals such as, for example, voltage differences between the signal lines. When receiving, the physical signals present on the bus system are received and converted into logic signals.
Classic CAN transceivers may be used for CAN FD, special modified transceivers may optionally contribute to a further increase in the bit rate or may take on additional functionalities.
The CAN FD protocol is described in a protocol specification entitled “CAN with Flexible Data-Rate Specification”, referred to below as the CAN FD specification, which may be downloaded on the Robert Bosch GmbH website http://www.semiconductors.bosch.de.